I have a question (part of my study) that I'm quite confused about. Before I go on, I'm not looking for a direct answer, but would love some guidance or feedback as to whether I am on the right track with my thoughts.

The question is posed thus "Predict what would happen to erythrocytes placed in a solution containing 300 mOsm/L urea (NH2)2CO and 300 mOsm/L sucrose (i.e. total of 600 mOsm/L of particles in one solution; and remember that this is NOT made by mixing equal volumes of 300 mOsm/L urea and 300 mOsm/L sucrose). Fully explain your answer"

The direction I am heading with my answer is as follows: Sucrose (glucose and fructose) and urea are all osmotically active, and the concentrations of these substances within the erythrocyte is lower than that in solution. They will therefore diffuse down the individual concentration gradients and induce water absorption or osmosis by the erythrocyte. This will cause the erythrocyte to swell and perhaps rupture (lyse). The individual concentration gradients of approx 300 mOsm urea, 150 mOsm glucose, 150 mOsm fructose - do not summate to provide a hypertonic solution which would cause crenation of they erythrocyte.

Just some cautionary advice. You seem to think that because sucrose is composed of glucose and fructose that 1 molecule of sucrose dissolves in water to give one molecule of glucose and one molecule of fructose. That isn't the case (ignoring the possibility of slow spontaneous hydrolysis). One molecule of sucrose yields one molecule of sucrose-as-solute when dissolved in water.

The other thing to consider is that urea can cross the RBC membrane while sucrose cannot.

Lastly, stop and think what you're doing to concentrations of solute when you're mixing two solutions together. What happens to the concentration of urea when you mix it with a solution of sucrose (and vice versa)?

I should add, you don't have to make the 600 mOsm/L solution by mixing a solution of urea with a solution of sucrose. You can also do it by adding solid urea and solid sucrose (in the correct amounts) to a final volume in water and get the same thing. Your "how to do it" descriptions are different for each case, that's all.

Many thanks blcr11, you have eliminated some of my confusion, but I still have a little ways to go I think before I totally get my head around this.

So to change the way I was thinking more to:

The mixed solution is 600 mOsm and is hyperosmotic to the cell at 300 mOsm (this means the cell would release water to its surroundings if osmolarity was the only consideration). This is where I am struggling I don't think I fully understand the difference between osmolarity and osmolality.

Because the RBC membrane is permeable to urea and this is diffused into the cell, this is ignored when considering tonicity?

The sucrose solutes however are non-penetrating particles (i.e. the RBC membrane is impermeable to sucrose) and is therefore the only solutes in the solution that are considered.

0.3m sucrose solution has an Osm of 300mOsm and this is the same as the osmolality and osmotic pressure of the erythrocyte and as such is isotonic. The cell will neither lose or gain water.

Therefore there is no net change in the shape of the cell?

Am I getting warmer?

Also is it safe to assume that tonicity is the only part of the process that does not take into account the solutes that are non-permeable?

Your confusion isn't osmolarity vs osmolality, though, it's with the difference between those two ideas and tonicity. The only difference between the first two is the units; osmolarity is moles solute per liter of solution while osmolality is moles solute per Kg of solution. Tonicity is a measure of osmotic pressure relative to a membrane (here the RBC membrane) or solution--often tonicity will be referred to blood or plasma depending on the use.

Osmolarity doesn't take into account if a solute will cross the membrane.

Tonicity does.

Therefore the 300mOsm sucrose is isotonic to the cell, and is taken into acount. The urea isn't considered with regard to tonicity as it freely diffuses. Only non-penetrating particles are considered in tonicity!

Hehe, thats gotta be close *wink*

Thanks again blcr11!!!

Talk about brain-stretching stuff, I love the challenge of trying to work some of this stuff out - but occasionally I think about things for tooo long or have totally the "wrong end of the stick" and have to surrender and get help! You're help has been invaluable!